Ji-woo, an outcast in a prestigious private high school, does not fit in due to different social backgrounds than his well-to-do classmates. One day, he meets Hak-sung, the school’s security guard who is actually a mathematical genius defected from North Korea, now living with his past shut away. Ji-woo asks Hak-sung to teach him math, and although reluctant at first, Hak-sung eventually agrees to. Finding each other in their most difficult times, Ji-woo and Hak-sung open up to one another during their special math lessons, but their friendship is at risk when Ji-woo is framed for an incident in school, and Hak-sung is faced with people prying on his past.